It is a known problem that, since the suspension cables of elevators can change their length in time or due to temperature changes and because the winch that winds these cables has some errors due to slips or other unpredictable events, the exact position of an elevator car can not be determined solely by the positioning of the suspension system. This means that even if a direct relation exists between the number of rotations of the winch and the height of the car, several factors can affect this relation. For this reason, a dedicated positioning system is required to be able to precisely determine the actual position of the elevator car within the shaft. Such systems must be independent from the suspension system in that the detection should not rely on the length of the cable wound up or on the rotational position of the winch because this might lead to serious errors ranging from a few centimeters up to meters if very long suspension cables are used.
Several approaches are known in the art to determine the exact position of an elevator car. One of these approaches suggests the use of a laser or other strong light source mounted on the car and a detector mounted at one of the shaft's ends and to measure the time needed for the light beam to travel from the emitter to the detector. Based on this measured time and knowing the propagation speed of the signal, light in most cases, one can determine the distance between the two, thus the position of the elevator car. The same principle works the same way if a light source is mounted at an end of the elevator shaft and a mirror is placed on the elevator cabin car to reflect the light back to the sensor. However, in both cases several disadvantages and difficulties arise: it is often a problem to guarantee a clear line of sight between the detector and the light source since the space between the ends of the elevator shaft and the elevator is usually occupied by the suspension cables, communication or power cables and other elements of the elevator system. A further problem is that in very tall shafts (high rise elevators) even a small vibration of the car may cause significant deviation of the light beam rendering the detection of the beam and thus of the car's position unreliable. Ensuring the cleanliness of the light source, the detector and in some cases the reflective mirror might also become a problem in certain cases.
Some elevator positioning systems use marked belts, or tapes that run parallel to the path of the car and a fixed scanner counts the number of markings that pass it as the car moves. However, such systems can only detect relative movement of the car and not absolute position and by that an error in the scanning can pass undetected for an extended period of time.
A different approach is described in U.S. Pat. No. 6,435,315. In this approach a code rail is mounted on a sidewall of the elevator shaft adjacent to the path of travel of the car that contains optically readable indicia and a camera, mounted upon the car, scanning the code rail indicia to determine the location of the car within the shaft. However, even if this system may work well in most cases, it is rather complicated in construction. The concept of this system requires that the detection camera to be mounted on the car itself which means that there is a need of some sort of communication between the position detection system in the car and the control system of the elevator motors in the shaft. This communication can be done by a wired communication line or by radio communication. Both of these have significant drawbacks. On one hand having additional cables between the shaft and car can be problematic because of the presence of the suspension ropes and other moving elements. In open or glass covered shafts it is esthetically undesirable to have additional cables hanging. On the other hand, radio communication between the car and the control system of the elevator motors in the shaft requires additional components and electric energy. Interferences with other radio devices or even intentional jamming of the signal can render the system unreliable.